DC to DC converters are often used in association with rectified and filtered AC line voltage. Converters that utilize a line voltage which has been rectified and filtered without the use of a transformer are referred to as "off-line" converters. In such converters, the input voltage often exceeds 200 volts due to peaks in the ripple voltage. Use of an off-line converter is desirable, however, as it eliminates the need for a transformer in the input stage. On the other hand, the off-line converter has a disadvantage in that it places a voltage stress on its switching network which is equal to twice the input voltage.
One typical off-line converter design utilizes a four transistor bridge and a transformer to reduce the 200 volt input to a desirable level. The four transistor bridge is used to reduce the voltage stress generally applied across its switching network. The disadvantage of this arrangement is that the four power switches require associated drive circuitry which is often complex.
Another off-line converter design utilizes a half bridge configuration with a pair of capacitors and a transformer to accomplish voltage conversion. This arrangement has the disadvantage of requiring two large capacitors with high ripple currents.
A typical push-pull transformer used in a converter circuit generally places a voltage equal to twice the input voltage across the emitter to collector junction of the transistor switches. This can create a limiting design factor when high DC input voltages are to be converted. The design is further limited by the fact that high current spikes created by overlapping of the transistors as they switch on and off add to the stress across the transistors.
For a discussion of the three basic types of DC to DC converters including a flyback converter, a forward converter and a push-pull converter, see van der Poel, Jan M., "Pick The Right DC/DC Converter", Electronic Design, June 7, 1978, pp. 104-108. This article mentions the problem of twice the input voltage across the transistors at page 106.